AlGaN/GaN FET effect transistor), a sort of compound semiconductor device making use of hetero-junction of AlGaN/GaN representing nitride compound semiconductor, and using a GaN layer as a channel layer, has vigorously been developed in these years, typically as found in Japanese Laid-Open Patent Publication Nos. 2007-329483, 2008-270521 and 2008-270794, and in International Publication Pamphlet No. WO 2007/108055. GaN is characterized by its wide band gap, high breakdown field strength and large saturation electron velocity, and is a great hope for material composing semiconductors capable of high-voltage and high-output operation. For this reason, the AlGaN/GaN FET is now under vigorous development, in pursuit of high-efficiency switching device for the next generation.